This doesn't seem strictly doable with out of the box SharePoint features.
As per my comment, I don't get what is the final effect that you want to accomplish
you need to insert the invoiced amount on the PO Master and DISPLAY it on the Invoice Master, insert on Invoice master and display on PO master or do you need to support insert/edit of the value on both ends of the relation?
Some of the requirement above could be achieved by using a "Multi-column" lookup. Ootb SharePoint doesn't provide such functionality, so you would have to resort to some third party solutions, like http://spfields.codeplex.com/ or something similar.
This won't still resolve the second problem - the calculated column. If the change in the calculated value is triggered by a change of the value in the other list (Invoice Master) your best hope is to implement a custom event receiver on the list: when a value change on one of the list, use the PO Number to find the related item on the other list and update your fields. Notice that you may need to disable event firing while performing the update to avoid an infinite cycle of event receiver triggering.
I will try to elaborate more if needed, please fell free to leave a notice here or on the original question.